This invention relates generally to clay processing, and more specifically relates to methods for increasing the brightness and whiteness of kaolin clays.
The commercial value of a refined kaolin is in good part a function of the brightness and whiteness of the product obtained from the refining processes to which a crude kaolin is commonly subjected in the course of yielding an end product. One of the principal sources of discoloring contaminants in the crude clay takes the form of insoluble oxides of iron. Thus a commonly used technique for removing the said contaminants involves forming the clay into an aqueous slurry, acidifying the slurry to a pH of the order of 3.0 to 4.0, and adding a slurry-soluble salt of hydrosulfurous acid. The general objective of this operation is to provide S.sub.2 O.sub.4.sup.= ion which acts as a reductive leaching agent. In particular such ion functions to reduce the ferric compounds present in the slurry to ferrous form, the latter being readily soluble and therefore removable by subsequent washing, dewatering, and filtering operations.
In the past zinc or sodium salts of hydrosulfurous acid have represented the most common materials used for the aforementioned leaching purposes. Both of these salts, however, have distinct disadvantages. While zinc hydrosulfite, for example, is highly effective for the leaching operation, its use results in waste water discharge displaying unacceptably high content of zinc ion. Similarly its use results in residual zinc ion in the processed clay, the actual residual content being a function of the efficiency of the thickening and washing process. Where such clays are later utilized in certain paper mill operations, the mill discharge water can contain zinc ion levels sufficiently high to endanger certain of the animal species in the streams or rivers into which such water passes.
The residual zinc ion and associated salts present in the treated clay can, further, result in poor rheological behavior of the clays. In particular, the effect can manifest itself through high initial Brookfield viscosities, and in highly unstable slurries. Thickening of the slurries arising from such cause is undesirable, in that the clay manufacturer often desires to ship in slurry form; and, secondly, the consumer of the materials may desire to store the clay as a slurry -- even where it is initially provided to him in a dry state.
The toxicity and rheological problems associated with zinc are obviated by use of sodium hydrosulfite. However, the latter compound is unstable, is possibly less effective in use than corresponding zinc compounds, and is furthermore in relatively short supply.
It may be noted in this connection that within recent years, a process utilizing sodium borohydride for production of sodium hydrosulfite has found favor in various applications -- including bleaching applications. One of these processes, for example, the "Borol" process (a trademark of Ventron Corp., Beverly, Mass.) uses a solution including sodium borohydride and sodium hydroxide. Sulphur dioxide gas is made to react with the Borol solution in a special reaction chamber, to produce liquid sodium hydrosulfite.
In the usual technique employed with Borol and similar solutions, the components are reacted to produce the hydrosulfite, which is then stored for later use. As already suggested, however, storage of liquid sodium hydrosulfite is undesirable because of decomposition taking place due to the effect of temperature, pH, inadvertent aeration, and self-decomposition resulting from the presence of impurities -- such as bisulfites produced during the generation reaction. Indeed in a typical instance it has been found that a sodium hydrosulfite solution produced from powder and having an initial 15.4% concentration of the hydrosulfite, will decompose approximately 35% in five days when maintained at 30.degree.C. At 35.degree.C the decomposition will amount to almost 50% in only 3 days. Furthermore, during transfer of the sodium hydrosulfite to the reaction vessel in which leaching is to occur, and during addition and subsequent mixing, further decomposition can occur, in accordance with one or more of the following equations: EQU a. 2Na.sub.2 S.sub.2 O.sub.4 + H.sub.2 O .fwdarw. Na.sub.2 S.sub.2 O.sub.3 + 2NaHSO.sub.3 EQU b. 2Na.sub.2 S.sub.2 O.sub.4 .sup.H.sup.+ Na.sub.2 S.sub.2 O.sub.5 +Na.sub.2 S.sub. 2 O.sub.3 EQU c. Na.sub.2 S.sub.2 O.sub.4 .sup.H.sup.+ NaOH + 2H.sub.2 SO.sub.2 EQU d. 2H.sub.2 SO.sub.2 .fwdarw. HSOH + H.sub.2 SO.sub.3 EQU e. HSOH .fwdarw. various products EQU f. Na.sub.2 S.sub.2 O.sub.4 .sup.111 O.sup.] 2NaHSO.sub.3
Thus reaction (f) above, becomes particularly pertinent in that air-derived oxygen present in the clay stream and introduced during the mixing is very detrimental, leading to a rapid decomposition to sodium bisulfite. Also of considerable pertinence for present purposes are reactions (b) and (c), which show the dramatic effect of pH. The lower the pH, in particular, the more rapid decomposition, so that at a pH of 0.8, for example, sodium hydrosulfite solutions have a half life of approximately 2 minutes at 25.degree.C.
In an attempt to eliminate the problem of advance preparation of sodium hydrosulfite solutions, it has been proposed in British Pat. No. 1,039,960, to initially treat an aqueous clay slurry with a water soluble sulfite, hydrogen sulfite or disulfite, and/or sulphur dioxide, or sulfurous acid. The said patent goes on to prescribe that an alkali metal borohydride be subsequently added to the slurry, after which the pH is adjusted to a value between 2.5 and 4. A primary difficulty with this approach is that the very addition of the aforementioned sulfites, and particularly of sulphur dioxide, lowers the pH in the slurry so that conditions favorable to decomposition of hydrosulfites are present before, in fact, the borohydride reactant is added.
A related teaching is found in U.S. Pat. No. 3,290,161 to Fred R. Sheldon et al. Sheldon thus teaches that initially one adds to an aqueous clay slurry an alkali bisulfite at a pH of about 3 to 3.5, and subsequently adds a borohydride to the treated slurry. Again in this instance, the initially acidic conditions favor decomposition.